High-performance display devices, such as liquid crystal displays (LCDs) and plasma displays, are commonly used in various electronics, such as cell phones, laptops, electronic tablets, televisions, and computer monitors. Currently marketed display devices can employ one or more high-precision glass sheets, for example, as substrates for electronic circuit components, or as color filters, to name a few applications. The leading technology for making such high-quality glass substrates is the fusion draw process, developed by Corning Incorporated, and described, e.g., in U.S. Pat. Nos. 3,338,696 and 3,682,609, which are incorporated herein by reference in their entireties.
The fusion draw process can utilize a fusion draw machine (FDM) comprising a forming body (e.g., isopipe). The forming body can comprise an upper trough-shaped portion and a lower portion having a wedge-shaped cross-section with two major side surfaces (or forming surfaces) sloping downwardly to join at a root. During operation, the trough is filled with molten glass, e.g., glass having a viscosity ranging from about 16,000 to about 75,000 poise, which is allowed to flow over the trough side walls (or weirs) and down along the two forming surfaces as two glass ribbons, which ultimately converge at the root where they fuse together to form a unitary glass ribbon. The glass ribbon can thus have two pristine external surfaces that have not been exposed to the surface of the forming body. The ribbon can then be drawn down and cooled to form a glass sheet having a desired thickness and a pristine surface quality.
During the glass forming process, molten glass can be delivered to one end of the isopipe (“delivery end”) and can travel down the length of the isopipe while flowing over the weirs to an opposite end (“compression end”). Forming bodies such as isopipes are often constructed of refractory ceramic materials, such as zircon, zirconia, alumina, and the like, which can have a coefficient of thermal expansion (CTE) that can widely vary as compared to the CTE of metal components of the isopipe, such as the end cap and/or plow. For instance, platinum and platinum-containing alloys can expand about two times as much as zircon at elevated temperatures. The expansion differential between the two materials can cause gaps to form during operation.
At operating temperatures, gaps can form that are large enough for glass at lower viscosities to flow through, particularly during flushing procedures. The molten glass can then begin to collect in an end cap of the isopipe. Glass collected in the end cap can be inactive and relatively stable, but can eventually leak out if the end cap malfunctions. For instance, leaks in the end cap can be caused by contamination of the welds and/or deterioration of the metal, e.g., due to contact with certain materials such as SiC. In some instances, excess glass volume in the end cap can cause the metal to bulge and place stress on the weld lines and/or stretch already thinned areas of the end cap. Bulging of the end cap can also cause it to contact a structure enclosing the forming body, thereby forming holes in the end cap. Excessive glass volume in the end cap can also force it to slip off the forming body, or isopipe, entirely.
Glass leaking from the end cap on the compression end can flow down into the rest of the process, e.g., behind an edge director at the edges of the primary glass flow, and is referred to in the art as “gobs.” Gobs can collect in size around the edge rolls below the root of the isopipe and can interfere with the pulling action. The gobs can also break off and cause glass pieces to get pinched between the glass and the lower rolls, which can result in significant glass breakage. Moreover, depending on the rate of glass flow into the end cap and the subsequent flow rate out of a leak in the end cap, early repair of the forming body or surrounding equipment may be required.
The amount of glass that collects in the end cap can be dependent on various factors, such as the amount of time the glass is in a low viscosity state, e.g., less than about 35,000 poise, the tightness of fit of the end cap, the depth of the isopipe trough, the angle of the weirs (as machined or as a function of process down tilts), and/or the process temperature (which can affect the expansion difference between the materials). For example, an end cap may not be attached or sealed to the isopipe other than by a tight mechanical fit. A slot can thus exist between the end cap and the isopipe as large as about 0.04 cm (0.015″). Molten glass having a viscosity of less than about 35,000 poise can flow through a 0.04 cm slot. Moreover, due to the gradual decline of the weirs from the delivery end to the compression end at a constant angle, e.g., a 6 degree angle, the top surface of the isopipe at the end cap region can be below the head level of the glass at the compression end. This can provide additional pressure for the glass to flow through the gaps or slots. Once the glass flows through the gaps or slots, it can flow into the end cap and over the end of the isopipe, resulting in one or more of the disadvantages discussed above.
Previous attempts to limit equipment damage, production loss, and/or glass damage due to gobs have included implementation of gob collection devices within the manufacturing system. However, gob collecting can upset operating parameters, such as thermal and/or mass balance, particularly in the case of glass forming processes for thin (e.g., less than about 0.3 mm) glass sheets. Frequency of flushing can, for instance, be increased to compensate for glass conversions and/or liquidus devit issues in high-precision glasses. Processes using longer and/or more frequent flushing can suffer from increased frequency and/or amount of glass leakage. Thus, thermal impact due to the collection and removal of large end mass using conventional methods can be detrimental to the glass forming process.
Consumer demand for high-performance displays with ever growing size and image quality requirements drives the need for improved manufacturing processes for producing high-quality, high-precision glass sheets. Accordingly, it would be advantageous to provide methods and apparatuses for forming glass ribbons and sheets which can minimize glass defects and/or breakage, as well as reducing equipment damage and process instabilities. In various embodiments, the methods and apparatuses disclosed herein can minimize glass flow into the end cap and over the compression end of the forming body, as well as the formation of gobs, which can minimize or prevent production losses and equipment damage.